There are devices that are provided with an electromagnetic flow rate control valve in order to control the flow of hydraulic oil that is supplied to a power steering device of a vehicle.
A large power assist force becomes necessary when a vehicle is running in a low speed region, during steering wheel operation in a state where the vehicle is stopped, and the like. In this case, an opening of the electromagnetic flow rate control valve is made larger, and the flow rate of the hydraulic oil supplied to the power steering device is made larger.
In contrast, the opening of the electromagnetic flow rate control valve is made smaller when almost no assist force is necessary, such as when the vehicle is running at high speed. In this case, surplus hydraulic oil bypasses the electromagnetic flow rate control valve by a pressure compensation valve and escapes, aiming for a reduction in energy loss.
FIG. 4 is a graph that shows a relationship between a control current I of a conventional electromagnetic flow rate control valve and a flow rate Qc that is supplied to a power steering device. As shown in this graph, the flow rate supplied to the power steering device is set to decrease as the control current of the electromagnetic flow rate control valve increases.
Setting the valve opening to become larger when the control current of the electromagnetic flow rate control valve becomes smaller is done so that a large valve opening is maintained even for cases in which current cannot be supplied due to a fault or the like. The electromagnetic flow rate control valve thus maintains a large opening by a spring retention force when there is no control current, and hydraulic fluid supply to the power steering device is made possible, thus exhibiting power assistance. That is, a failsafe function can be maintained.
If the opening is set to become larger when the control current of the electromagnetic flow rate control valve is large, the opening will become smaller when the current cannot be supplied due to damage or the like, and sufficient power assistance cannot be achieved.
However, if a large current is supplied to a solenoid of the electromagnetic flow rate control valve, electric power consumption also increases. Moreover, a state where the solenoid of the electromagnetic flow rate control valve is excited by a large current is a state where the power assist force of the power steering device is not necessary, that is, a standby state. This type of state accounts for most of the time when the vehicle is running.
Accordingly, the total energy consumption by the electromagnetic flow rate control valve becomes large with conventional techniques.
An object of this invention is to provide a flow rate control device capable of maintaining a failsafe function while further reducing energy loss.